HADOOP-9291. enhance unit-test coverage of package o.a.h.metrics2 (Ivan A. Veselovsky via jeagles)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1534474 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd